The present invention relates to smoking articles and, more particularly, to cigarettes.
Conventional cigarettes include a rod of smokable material, usually shredded tobacco material, wrapped in a combustible, air permeable paper wrapper. A filter, such as an acetate tow filter, is often wrapped in a substantially air-impermeable plug wrap paper, and is attached to an end of a wrapped tobacco rod by tipping paper. Non-filter cigarettes are generally 70 mm in length. Filter cigarettes are generally either 85 mm in length, which length includes a filter between 20-25 mm in length, or 100 mm in length, which length includes a filter between 25 and 32 mm in length. Most cigarettes are approximately 8 mm in diameter.
When a cigarette is lit to form a coal at one end, the cigarette burns either through puffing, when air is drawn through the cigarette through the coal and mainstream smoke is formed, or through smoldering, when air is moved upward around the coal and sidestream smoke is formed. During puffing, peak temperatures of approximately 900.degree. C. are developed. It is generally understood that, in conventional cigarettes, about twice as much tobacco is burned in the interval between puffs compared with that consumed during puffing.
Various efforts have been directed to reducing sidestream smoke emanating from lit cigarettes. It is known that reducing the permeability of the cigarette paper tends to reduce the burn rate of the cigarette and, consequently, reduces sidestream smoke. However, it is also understood that a cigarette constructed from cigarette paper having a permeability below about 5 Coresta units may not sustain smoldering. U.K. Patent Application 2094130 teaches that sidestream smoke is reduced in cigarettes manufactured with cigarette paper having a permeability of no more than 3 Coresta units due to viscous flow (i.e., flow through the paper through openings inherent from the paper making process), as contrasted with permeability due to inertial forces, (i.e., permeability due to flow through perforations formed in the paper). As explained in that document at page 1, lines 18-19, the ratio of the coefficient of diffusion of oxygen through nitrogen in the paper and the thickness of the paper must fall within a certain range. The desired ratio depends to a certain extent on design factors of the smoking article, such as the type and form of the tobacco and the diameter of the smoking rod.
Another technique for reducing the sidestream smoke of cigarettes involves adding a burn modifier to the cigarette wrapper paper. Various compounds may be added to the cigarette wrapper paper to affect the sidestream smoke level. U.S. Pat. No. 5,152,304, for example, indicates that the use of an acidic salt added to the cigarette wrapper paper reduces sidestream smoke. At certain levels, these additives may impart their own note to the taste of a cigarette.
Yet another technique for reducing sidestream smoke in a cigarette involves providing an insulating, air permeable wrapper or tube around a tobacco rod. U.S. Pat. No. 5,105,838 teaches that a cigarette manufactured with a tube of air permeable insulating material, e.g., a ceramic insulating material surrounding a tobacco rod, produces reduced sidestream smoke levels. U.S. Pat. No. 4,915,117 teaches providing a cigarette including a thin ceramic sheet for holding the tobacco. U.S. Pat. No. 4,480,650 teaches a cigarette formed with a paper wrapper having a clay coating which burns at a slower rate than the tobacco ember so as to control the rate of heat transfer from the smoldering coal to a surrounding environment. See also U.S. Pat. No. 5,159,940 which utilizes a non-combustible, substantially cylindrical hollow ceramic sleeve in a smoking article which generates substantially no sidestream smoke. Japanese Patent Document 04-183,378, published Jun. 30, 1992, shows a cigarette within a pipe that will not self-sustain its smolder.
Accordingly, the search has continued for cigarettes exhibiting reduced sidestream smoke without adversely affecting the taste of the cigarette.